1. Field of the Invention
The object of the invention is a method defined in the preamble of claim 1, an arrangement defined in the preamble of claim 5 for attenuating noise in a space by generating antinoise, and a mobile station defined in the preamble of claim 10.
2. Description of the Prior Art
Noise absorbing and isolating materials are generally used for noise attenuation. In some rooms the material required for the attenuation occupies too much space or is otherwise difficult to locate. This has been the reason to develop active noise attenuation systems where the system measures the noise and supplies antinoise into the same room. The object is to use the antinoise to attenuate the noise caused by the noise source. In an ideal case the interference of the noise and the antinoise is zero.
There are known active adaptive noise attenuation systems which from a noise reference generate an antinoise signal to be supplied to a speaker. The operation of the system is adapted on the basis of the residual noise, so that the residual noise will be as low as possible. The residual noise is measured with an error microphone included in the system. The noise reference describes the unattenuated noise caused by the noise source, which is measured with a second microphone, or which is generated from the error signal obtained from the error microphone. Alternatively the noise reference is obtained with another transducer, such as an acceleration transducer which provides information about the noise source. Then the noise caused by the noise source is deduced from its movements.
The Filtered-X LMS (Least Mean Square) algorithm is the algorithm most commonly used in adaptive noise attenuation systems. The attenuation arrangement realised with the aid of this algorithm is shown in FIG. 1. With the LMS algorithm the object is to minimise the power of the residual noise. The estimating filter 1 contains the estimate C' of the acoustic response C of the space where the noise occurs. The acoustic response C of a space means that effect, which the space has on the sound supplied to the space. The signal r processed by the estimate filter 1 is supplied to the first input of the calculating unit 2. The residual noise signal e is supplied to the second input of the calculating unit. The calculating unit contains the LMS algorithm. The calculating unit 2 calculates a control signal, which controls the adaptive filter 3, whereby the calculation is based on the signals supplied to it and on predetermined constant values. The noise reference x is supplied to the input of the adaptive filter 3. The calculating unit 2 and the adaptive filter 3 form together a digital unit 6, which in the inputs and the outputs has A/D converters and D/A converters, respectively. The adaptive filter 3 is a digital filter, such as a FIR filter (Finite Impulse Response), whose coefficients are suitably modified by the calculating unit 2. The adaptive filter 3 provides a speaker signal ka, which changes in the space 4 according to the acoustic response C and which is added 5 to the noise occurring in the space. This results in the error signal or the residual noise e, which is supplied to the second input of the calculating unit 2.
When said algorithm is used the adaptive FIR filter supplies at its output EQU y[n]=w.sub.0 [n]X[n] +w.sub.1 [n]X[n-1] +w.sub.2 [n]x[n-2]+ (1)
where x[n] is the input signal or the noise reference at the moment n, and the filter coefficients w.sub.k are updated according to the formula EQU w.sub.k [n+1]=w.sub.k [n]-2 .mu.e[n]r[n-k] (2)
where n is the sampling moment of the digital filter, .mu. is a constant step length, e is the error signal, and r is the filtered reference signal x, which is filtered by the electroacoustic response estimating filter. In the formulas k is a finite number series from zero to the positive limit. A more comprehensive description of adaptive signal processing is presented in Widrow B., Stearns S. D., Adaptive Signal Processing, Prentice-Hall, 1985.
A problem of the known adaptive noise attenuation systems is that there is a substantial amount of residual noise which can be heard, and that the power required for the sound pressure which is needed to generate the antinoise is high compared to the attenuation result.